FIG. 1 shows a local mobile access network RLAM to which the present invention may be applied. This local network is comprised of a plurality of radio terminals BR each linked to an input of a multiplexer MUX-10 to MUX-13, here there are four. Each multiplexer MUX-10 to MUX 13 has its output linked to an input of a concentrator CTR-10, CTR-11 of which there are two in the network shown. Finally, each concentrator CTR-10, CTR-11 has its output linked to an input multiplex of a virtual circuit commutator VCX-10, an output multiplex of which is linked to a general network RG. Each element of the local network RLAM uses the the technology known under the name ATM (Asynchronous Transfer Mode).
A network of this type, which is given here by way of example, is a network which is fixed at three levels of hierarchy: multiplexer level, concentrator level, commutator level. It will be noted that the present invention does not only apply to networks having three hierarchy levels but also to networks having two or more than three levels of hierarchy.
In the present description, the concentration stage will be called the multiplexers MUX, the concentrators CTR and also the commutator(s) of virtual circuits VCX of the network.
In the present description, it is said that a mobile unit is linked to a local network RLAM when it passes under the radio coverage of one of the terminals BR of this network. By way of example, the coverage of such a network could be that of an average agglomeration. Always by way of example, several networks of this type can be juxtaposed to increase the geographic area covered. Thus, a regional or national network should amalgamate a more or less significant number of local access networks RLAM.
In this type of access network, the communications are managed in the commutator VCX. They can be local when they are established between mobile units linked to the local network RLAM in question, but they can also be outgoing when they are established between mobile units connected to the RLAM network in question and any terminal equipment of the general network RG. During its entire duration, a communication is identified in the local network by a same pair of virtual conduit and virtual channel identifiers VPI/VCI, even when the connections are consecutively modified to the movements of the mobile unit in question.
In a network of this type, a mobile unit which is already in communication and which is thus connected to one of the terminals BR can suddenly, when it is moved, be under the coverage of a second radio terminal BR and thus be momentarily connected to two terminals BR. It can thus be said that this mobile unit is in an "intercell hand-over" phase. In this phase, if the user traffic links are all established toward the two terminals and the two terminals to the commutator, the information transmitted by the mobile unit is received simultaneously by the terminals BR to which it is connected, then transmitted in the local network. The mobile unit itself receives information coming simultaneously from said terminals BR.
In order not to unnecessarily encumber the RLAM network, it is known to use equipment whose function is to filter the flow coming from a same mobile unit in a hand-over phase and to only maintain one, generally, the one which has the best quality. The filtered flow is thus released on the network. In reality, this equipment generally operates at the level of the ATM cells even if they could be retained by the macrodiversity operation, dependent on the quality of the radio reception, alternatively or not from the ATM cells coming respectively from different terminals.
It is noted that, when a mobile unit is under the coverage of several terminals, the radio connections generally have very unequal properties.
This equipment is called an OPM macrodiversity server and its filtering function is called macrodiversity.
In a local network, an OPM server can be seen as an OPM server or as a commutation unit. In the first case, it is connected to a concentration stage, such as a multiplexer, a concentration apparatus or a commutator such that it can be placed in derivation, as the one shown in FIG. 1, even though in the second case it is cut off from an access multiplex connection to a concentrator or a commutator, like the one shown in FIG. 4.
The present invention applies to two types of OPM server.
A mobile access network RLAM should be provided with at least one OPM server located on the general access network RG, that is, at the level of the main commutator VCX of the local network, in order that the macrodiversity operation may be performed on flows coming from any terminal of the RLAM network. However, for reasons of efficiency and optimization of the occupation of resources in the local network, it may be advisable to place an OPM server at various stages of concentration, indeed at each stage of concentration.
In fact, in a network having several hierarchy levels as described above, an OPM server placed at the lower hierarchy level enables filtering of the flows from a mobile unit which would be in a hand-over phase on two terminals connected to the same concentration stage present at this point. Similarly, an OPM server placed on a higher concentration stage level enables filtering of the flows from a mobile unit connected to two terminals connected to all of the concentration stages at a slightly lower level.
The sequence of events which generally result during an intercell hand-over phase with macrodiversity are as follows. The traffic passes up to the first terminal, called terminal A. The mobile unit in communication and connected to said terminal A confirms that the quality of the radio connection is better on another terminal B. It then establishes a signal channel in the RLAM local network on this new terminal B. The context of the connection is then transferred from terminal A to terminal B, i.e. information concerning the radio supports and ATM and the routing service. A user channel is then established in the local network on terminal B as a result of which the mobile traffic passes through both terminals A and B. This is the start of the macrodiversity phase. When the terminal cuts the traffic, it is the end of this phase. The user and signal channels on terminal A are then disconnected.
It is the object of the present invention to provide, in this context, a directional method for cells transmitted by a mobile unit to an OPM server for a given virtual connection.
A simple solution for solving the directional problem to an OPM server would be to consider that it will always be in service. Thus, any user traffic passing through the OPM server on condition that it separate the connections to be processed in macrodiversity and those which it agrees to let pass because they are not in a hand-over phase. Several disadvantages give reason to reject this approach. First, there is an increase in the complexity of the OPM server which is going to pass any communications in progress and which should be capable of managing a significant number of contexts. This results in a higher system cost, namely by using a significant number of memory elements. This architecture is then more sensitive to breakdowns and the consequence of an OPM server failure is that all communications are interrupted, whether they be in a hand-over phase or not. Finally, this architecture necessitates that the OPM server be placed in a commutation unit which may create problems, in particular the complexity of the system.